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Urbanisation made flooding from Hurricane Harvey 21 times as likely

The urban landscape can make flooding worse

REUTERS/Richard Carson

By Michael Le Page

The urbanisation of Houston made Hurricane Harvey dump even more water on the city – because the tall buildings pushed air upwards and caused more rain to fall. Overall, urbanisation increased the flooding risk by a whopping 2000 per cent.

The 2017 hurricane caused $125 billion in damage, making it the second costliest tropical storm ever. Now a study has used climate models to compare how much flooding a hurricane like Harvey produces in a virtual version of Houston as it is now, and how much when the city is replaced by farmland.

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What’s less appreciated is that the presence of high buildings can also increase how much rain falls during a storm. The larger surface “roughness” in urban areas created a “drag effect” on Hurricane Harvey that moved warm surface air further up into the atmosphere, thereby creating conditions favourable for cloud formation and precipitation, says team member Gabriele Villarini at the University of Iowa. “To a certain extent, this is kind of similar to clouds over mountains.”

These two factors – higher rainfall and increased run-off – together increased the risk of flooding during Hurricane Harvey 21-fold, the modelling study suggests. Villarini says he cannot say how much each contributed separately.

The land around Houston would have been forest and marshes originally, but Villarini thinks the results would have been similar if they had replaced farmland with forests in the models.

Previous studies of Harvey suggest that global warming likely increased its rainfall total by between 15 and 40 per cent. All in all, it seems Harvey was a very unnatural disaster, a catastrophe very much of our own making.